In conventional Non-Invasive Blood Pressure (NIBP) measurement technologies, a cuff is used to stress the blood vessel to block the blood flow, and then the pressure on the cuff is released gradually and a systolic pressure and a diastolic blood pressure can be measured by detecting the pulse of the blood vessel via stethoscope or another detecting device. However, because the conventional NIBP sphygmomanometer needs to stress the blood vessel with the cuff to measure the blood pressure, as a result, the size of the sphygmomanometer and its accessories may be too big to carry, and the user cannot use the sphygmomanometer to measure the blood pressure continuously for long tune. Furthermore, in conventional NIBP measurement technologies, the cuff is inflated to stress the blood vessel, the sphygmomanometer may generate noise and the user may feel uncomfortable.
Therefore, cuff-less measurement technologies are proposed, wherein in cuff-less measurement technologies, blood pressure is measured according to the pulse transit time (PTT) or pulse wave velocity (PWV) calculated through an Electrocardiogram (ECG) with Photoplethysmography (PPG). However, the EGG needs to use an electrode to touch the skin to measure the weak voltage variety in human skin, and the PPG detects the signals of the blood vessel through the photodetector and is easily affected by the background light source. Therefore, for the usage of the EGG and PGG, the EGG and PGG needs to tightly touch the skin to avoid the background noise which may lead to the degradation of the signal-to-noise-ratio (SNR). Therefore, although the cuff-less measurement technologies may be able to reduce the discomfort generated by the cuff of the conventional NIBP sphygmomanometer, the cuff-less measurement technologies still cannot avoid the need to tightly touch the skin and cannot avoid the errors generated because of the different lengths of blood vessels. In addition, with cuff-less measurement technologies, the user also needs to wear additional accessories which may lead to inconvenience when measuring blood pressure.